Constant-voltage power circuit with fold back current limiting capability

ABSTRACT

A constant-voltage power circuit incorporating an over-current protection circuit having a fold back current limiting capability operative through low input voltages, and being capable of arbitrarily setting current outputs. The constant-voltage power supply circuit is configured so that a voltage output with small amplitude from a differential amplifier circuit is subjected to amplitude expansion to be amplified to range fully from the ground potential approximately to the input voltage by means of an amplitude expansion circuit, which includes an inverter consisting of an NMOS transistor and a resistor, and that the voltage resulting from the amplitude expansion is subsequently input to the gate of a PMOS transistor configured to directly control an output transistor.

This application claims priority to Japanese Patent Application No.2005-132446, filed with the Japanese Patent Office on Apr. 28, 2005, theentire contents of which are hereby incorporated by reference.

FIELD OF INVENTION

The invention generally relates to constant-voltage power circuits, andmore specifically to a constant-voltage power circuit incorporating anover-current protection circuit having a fold back current limitingcapability operative through low input voltages.

BACKGROUND OF INVENTION

In order to implement overcurrent protection for constant-voltage powersupply circuits employing series regulators, overcurrent protectioncircuits have been widely used such as, for example, a current limitingcircuit configured to prevent a current output from exceeding apredetermined current, and an overcurrent protection circuit configuredto limit a current output under short circuit conditions.

The overcurrent protection circuit is characterized by the so-calledfold back current limiting capability, which provides better protectionthan conventional current limiting because as a load resistancedecreases below a predetermined value, both the voltage and currentdecrease simultaneously along a characteristic foldback locus.

FIG. 3 is a diagrammatic circuit diagram illustrating a knownconstant-voltage power supply circuit incorporating an overcurrentprotection circuit.

Referring to FIG. 3, the known constant-voltage power supply circuit 100includes a constant-voltage circuit 101 and an over-current protectioncircuit 102.

The constant-voltage circuit 101 includes a reference voltage generatingcircuit 111 configured to generate and output a predetermined referencevoltage Vref, an error amplification circuit AMP, an output transistorM101, and resistors R101 and R102, configured to generate and output apartial voltage VFB, which is obtained by dividing an output voltage Vo.

In addition, the over-current protection circuit 102 includes PMOStransistors M102, M103, M106, and M107; depletion-type NMOS transistorsM104 and M105; a resistor R103, and a bias current source 112.

In the case when a current output from the output transistor M101 issmaller than a predetermined current value for exerting over-currentprotection, the drain current of the current detection transistor M2 isrelatively small and the voltage drop by the resistor R103 becomessmaller than the sum of the partial voltage VFB at the junction betweenthe resistors R101 and R102, and an offset voltage Vof. This makes thedepletion-type NMOS transistor M105 turned off.

As a result, the gate voltage of the depletion-type NMOS transistor M105is brought approximately to the input voltage Vin, the PMOS transistorM103 is turned off, and no over-current protection is performed.

When the current output from the output transistor M101 reaches thepredetermined current value for exerting over-current protection, thevoltage drop by the resistor R3 is brought to be equal to the sum of thepartial voltage VFB and the offset voltage Vof.

As a result, the depletion-type NMOS transistor M105 is turned on, itsdrain voltage is decreased, and the PMOS transistor M103 is turned on.

By turning the PMOS transistor M103 on, the gate voltage of the outputtransistor M101 is elevated, the increase in the current output from theoutput transistor M101 is suppressed, and the output voltage Vodecreases.

Since the gate voltage of the depletion-type NMOS transistor M104decreases with the decrease in the output voltage Vo, the over-currentprotection can be made operative for a small voltage drop by theresistor R3, and the output current io decreases with the decrease inthe output voltage Vo.

When the output terminal OUT is short-circuited to the ground potential,the gate voltage of the depletion-type NMOS transistor M105 becomesequal to offset the voltage Vof.

During the short-circuit of the output terminal OUT, the magnitude ofthe current output from the output transistor M101 as the short-circuitcurrent, is equal to the product of the current through the resistor R3multiplied by the ratio of current between the output transistor M101and the current detection transistor M102. That is, the magnitude ofshort-circuit current can be set by the values of the offset voltage Vofand the resistance of resistor R3.

In the abovementioned over-current protection circuit 102, however, thedepletion-type NMOS transistor is used as an input component of theerror amplification circuit in order to make the error amplificationcircuit operative at the voltages as small as approximately 0 (zero) V.

Since the depletion-type NMOS transistor has a gate voltage smaller thanthe source voltage in the range of small drain current, a certain degreeof source voltage is required for the depletion-type NMOS transistorM104 or M105 even after the gate voltage of depletion-type NMOStransistor M104 is decreased to 0 V at short-circuit conditions.

Therefore, a difficulty with this over-current protection circuit isthat the drain voltage of the depletion-type NMOS transistor M105 cannotbe decreased to sufficiently low.

Along with the recent efforts to reduce power consumption in variousequipments, circuit voltages also have been decreasing as typicallyevidenced by compact, portable electronic devices.

For example, the voltages input to the series regulated constant-voltagepower circuits have been supplied recently with a minimum necessaryvoltage after processed once with a step-down DC-DC converter.

Moreover, the input voltage itself has shifted to as low asapproximately 1.5 V, and problems are caused such as, for example, thegate voltage of PMOS transistor M103 cannot decrease low enough and thisin turn brings the over-current protection circuit non-operative as longas the depletion-type NMOS transistor is used as the input component ofthe error amplification circuit, as mentioned earlier.

In order to address this problem and provide an over-current protectioncircuit capable of operating at low voltages, another known circuit isdisclosed as illustrated in FIG. 4 (Japanese Laid-Open PatentApplication No. 2004-118411, for example).

The components in the over-current protection circuit of FIG. 4 that aresimilar to those of the constant-voltage power supply circuit describedearlier in reference to FIG. 3 are shown with identical numericalrepresentations.

Referring to FIG. 4, the over-current protection circuit 100 includesPMOS transistors M112, M113, M114, and M115; an NMOS transistor M118;and resistors R113 and R114.

The PMOS transistors M112 serves as a current detection transistor,which is configured to output a current proportional to the outputcurrent from an output transistor M101.

The over-current protection circuit of FIG. 4 has a device structuresimilar to the constant-voltage power supply circuit of FIG. 3, with theexception that PMOS transistors M114 and M115 are included in the inputcircuit of error amplification circuit, and that the drain current ofthe detection transistor M112 serves as a bias current of the erroramplification circuit.

The drain current of the detection transistor M112 is then distributedto PMOS transistors M114 and M115, and converted into a voltage with theresistor R113.

In the case when a current output from the output transistor M101 issmaller than a predetermined current value for exerting over-currentprotection, the drain current of the detection transistor M112 passeswhile evenly shared through PMOS transistors M114 and M115. Since thedrain current is small, the voltage drop by the resistor R113 is alsosmall.

As a result, the PMOS transistor M113 is turned off and no over-currentprotection is performed.

When the current output from the output transistor M101 reaches thepredetermined current value for exerting over-current protection, thevoltage drop by the resistor R113 reaches the threshold voltage of NMOStransistor M118 and the transistor is turned on.

By turning the NMOS transistor M118 on, the gate voltage of the PMOStransistor M113 decreases to be turned on, the gate voltage of theoutput transistor M101 is elevated. As a result, the increase in thecurrent output from the output transistor M101 is suppressed and theoutput voltage Vo decreases.

Since the gate voltage of PMOS transistor M114 decreases along with thedecrease in the output voltage Vo, the current through the PMOStransistor M114 and accordingly through the resistor R113 increases. Theoutput current from the output transistor M101 decreases therefore withthe decrease in the output voltage Vo.

However, a difficulty with the circuit configuration of FIG. 4 is thatthe output current from the output transistor M101 is one half of theoutput current maximum under short-circuit conditions, and it is notfeasible for the value of output current be set arbitrarily.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide aconstant-voltage power circuit having most, if not all, of theadvantages and features of similarly employed power circuits, whilereducing or eliminating many of the aforementioned disadvantages.

It is another object to provide a constant-voltage power circuitincorporating an over-current protection circuit having a fold backcurrent limiting capability operative through low input voltages, andcapable of arbitrarily setting the values of current output.

The following description is a synopsis of only selected features andattributes of the present disclosure. A more complete descriptionthereof is found below in the section entitled “Description of thePreferred Embodiments.”

The above and other objects of the invention are achieved by providing aconstant-voltage power supply circuit configured to convert a voltage,which is input to an input terminal, into a predetermined constantvoltage, and to output from an output terminal.

The constant-voltage power supply circuit includes at least

an output transistor configured to output a current from the inputterminal to the output terminal according to a signal input to a controlelectrode;

an output voltage control unit configured to generate the predeterminedreference voltage, generate a voltage proportional to the voltage,amplify the difference between the predetermined reference voltage andthe voltage proportional to the voltage to obtain an amplifieddifference, and input the amplified difference to the control electrode;and

an over-current protection circuit configured to perform an controloperation such that, in the case when the current output from the outputterminal reaches a predetermined current value or greater for a ratedoutput voltage, the first current as well as the first voltage aredecreased, and that, in the case when the first voltage from the outputterminal is decreased to a ground potential, a predeterminedshort-circuit current is output from the output terminal.

In addition, the over-current protection circuit includes at least

(a) a current detection transistor configured to detect the currentoutput from the output transistor, and to output a current proportionalto the current input from the output transistor,

(b) a current-voltage conversion circuit configured to convert thecurrent output from the current detection transistor into a voltage toobtain a converted voltage;

(c) an error amplification circuit provided with first and second inputterminals which are input with the current proportional to the currentoutput from the output transistor and the converted voltage,respectively;

(d) an amplitude expander configured to expand an amplitude of thevoltage output from the error amplification circuit, so as to range fromthe ground potential approximately to the input voltage; and

(e) a control circuit configured to control an operation of the outputtransistor according to an output signal from the amplitude expander.

Moreover, the error amplification circuit is formed including an inputcircuit which is supplied with an offset voltage such that the value ofthe predetermined short-circuit current is obtained as positive.

With this device configuration, the constant-voltage power circuit ofthe invention is capable of performing over-current protectioncharacterized by the fold back current limiting capability operativethrough low input voltages.

These and other features and advantages of the invention will be moreclearly seen from the following detailed description of the inventionwhich is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, like reference numerals will be used to referto like elements, in which:

FIG. 1 is a diagrammatic circuit diagram illustrating a constant-voltagepower supply circuit according to an exemplary embodiment of theinvention;

FIG. 2 is a diagrammatic circuit diagram illustrating a constant-voltagepower supply circuit according to another exemplary embodiment of theinvention;

FIG. 3 is a diagrammatic circuit diagram illustrating a knownconstant-voltage power supply circuit incorporating an overcurrentprotection circuit; and

FIG. 4 is a diagrammatic circuit diagram illustrating a knownover-current protection circuit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the detailed description which follows, specific embodiments aredescribed on a constant-voltage power circuit incorporating anover-current protection circuit having a fold back current limitingcapability.

It is understood, however, that the present disclosure is not limited tothese embodiments. For example, it is appreciated that the present powercircuit and its structure may also be adaptable to various circuits.Other embodiments will be apparent to those skilled in the art uponreading the following description.

In addition, in the description that follows specific terminology isused in many instances for the sake of clarity. However, the disclosureof this patent specification is not intended to be limited to thespecific terminology so selected and it is to be understood that eachspecific element includes all technical equivalents that operate in asimilar manner.

According to a general example in the present disclosure, aconstant-voltage power circuit incorporating an over-current protectioncircuit is provided, being configured to convert a voltage which isinput to an input terminal, into a predetermined constant voltage, andto output from an output terminal.

The constant-voltage power supply circuit may include at least an outputtransistor, an output voltage control unit, and an over-currentprotection circuit.

The output transistor is configured to output a first current from theinput terminal to the output terminal according to a signal input to acontrol electrode.

The output voltage control unit is configured to generate thepredetermined reference voltage, generate a second voltage proportionalto the first voltage, amplify the difference between the predeterminedreference voltage and the second voltage proportional to the firstvoltage to thereby obtain an amplified difference, and input theamplified difference to the control electrode.

The over-current protection circuit is configured to perform an controloperation such that, in the case when the first current output from theoutput terminal reaches a predetermined current value or greater for arated output voltage, the first current as well as the first voltage aredecreased, and that, in the case when the first voltage from the outputterminal is decreased to a ground potential, a predeterminedshort-circuit current is output from the output terminal.

In addition, the over-current protection circuit includes at least

(a) a current detection transistor configured to detect the firstcurrent from the output transistor, and to output a second currentproportional to the first current input from the output transistor, inwhich a control electrode of the current detection transistor isconnected to the control electrode of the output transistor and acurrent input terminal of the current detection transistor is connectedto the input terminal as well as the output transistor,

(b) a current-voltage conversion circuit configured to convert thesecond current from the current detection transistor into a thirdvoltage to obtain a converted voltage;

(c) an error amplification circuit provided with first and second inputterminals which are input with the second current proportional to thefirst current output from the output transistor and the convertedvoltage, respectively;

(d) an amplitude expander configured to expand the amplitude of a fourthvoltage output from the error amplification circuit,

so as to range from the ground potential approximately to the inputvoltage; and

(e) a control circuit configured to control an operation of the outputtransistor according to an output signal from the amplitude expander.

Specifically, input and output terminals of the amplitude expander areconnected to corresponding terminals of the error amplification circuitand the control circuit, respectively, to thereby form an invertercircuit.

In addition, the error amplification circuit includes first and secondtransistors, which form first and second halves of a differential pair,to thereby form first and second diodes as first and second loads of thefirst and second transistors, respectively.

Still in addition, the amplitude expander includes

-   -   (i) a third transistor configured to form a first current mirror        circuit in combination with the first transistor, in which the        control electrode of the first transistor is input with the        second current proportional to the first current,    -   (ii) a fourth transistor configured to form a second current        mirror circuit in combination with the second transistor, in        which an control electrode of the second transistor is input        with the converted voltage; and    -   (iii) a third current mirror circuit configured to supply a        third current from the fourth transistor to the third        transistor.

In addition, the control circuit is configured to control an operationof the output transistor according to a fifth voltage at the junctionbetween the third transistor and the third current mirror circuit.

Moreover, the error amplification circuit is formed including an inputcircuit which is supplied with an offset voltage such that thepredetermined short-circuit current is obtained as a positive value.

Having described the present disclosure in general, several preferredembodiments of the constant-voltage power supply circuit will bedescribed herein below according to the present invention with referenceto FIGS. 1 and 2.

FIG. 1 is a diagrammatic circuit diagram illustrating a constant-voltagepower supply circuit according to a first embodiment disclosed herein.

Referring to FIG. 1, a constant-voltage power supply circuit 1 isconfigured to convert an input voltage Vin, which is input to an inputterminal IN, into a predetermined constant voltage, and subsequentlyoutput as an output voltage Vo from an output terminal OUT.

The constant-voltage power supply circuit 1 may be integrated to be asingle IC (integrated circuit) device.

The constant-voltage power supply circuit 1 includes at least aconstant-voltage circuit 2 configured to convert an input voltage Vininto a predetermined constant voltage to subsequently be output as anoutput voltage Vo from an output terminal OUT, and an over-currentprotection circuit 3 configured to have a fold back current limitingcapability, in that this current limiting capability is characterized bythe function of the protection circuit 3 such that, in the case when anoutput current io, which is output from the output terminal OUT, isequal to or larger than a predetermined value of over-current protectioncurrent, the output current io is controlled to be suitably suppressedby decreasing the output voltage Vo.

The constant-voltage circuit 2 includes a reference voltage generatingcircuit 11 configured to generate and output a predetermined referencevoltage Vref; resistors R1 and R2 for use in detecting output voltages,configured to generate and output a partial voltage VFB which isobtained by dividing the output voltage Vo; an output transistor M1,consisting of a PMOS transistor, configured to control a current outputio to the output terminal OUT according to a signal input to a gate; andan error amplification circuit A1 configured to control the operation ofthe output transistor M1 such that the partial voltage VFB is equal tothe reference voltage Vref.

The over-current protection circuit 3 includes a differential amplifiercircuit A2, PMOS transistors M2 and M3, an NMOS transistor M4, andresistors R3 and R4.

In addition, the differential amplifier circuit A2 includes PMOStransistors M5 and M6, NMOS transistors M7 and M8, and a constantcurrent source 12.

In this structure, the error amplification circuit A1, reference voltagegenerating circuit 11, and resistors R1 and R2 constitute an outputvoltage control unit. And, the PMOS transistor M2 serves as a currentdetection transistor, the resistor R3 as a current-voltage conversioncircuit, and the NMOS transistor M4 and resistor R4 as an amplitudeexpander, respectively.

In the constant-voltage circuit 2, the output transistor M1 is connectedbetween the input terminal IN and the output terminal OUT, and theresistors R1 and R2 are connected serially between the output terminalOUT and the ground.

In the error amplification circuit A1, the gate of PMOS transistor M1 isconnected to the output terminal, the partial voltage VFB is input tothe non-inverting terminal, and the reference voltage Vref is input tothe inverting terminal.

In the over-current protection circuit 3, the source of PMOS transistorM2 is connected to the input terminal IN, and the gate of PMOStransistor M2 is connected to the gate of the output transistor M1.

In addition, the resistors R3 is connected between the drain of PMOStransistor M2 and the ground, and the joint of PMOS transistor M2 andthe resistors R4 is connected to the gate of PMOS transistor M6 as oneinput of the differential amplifier circuit A2.

Still in addition, the resistor R4 and NMOS transistor M4 are connectedserially between the input terminal IN and the ground; and the output ofthe differential amplifier circuit A2, i.e., the joint of PMOStransistor M5 and NMOS transistor M7, is connected to the of NMOStransistor M4; and the partial voltage VFB is input to the other inputof the differential amplifier circuit A2.

Moreover, the PMOS transistor M3 is connected between the input terminalIN and the output transistor M1, and the gate of PMOS transistor M3 isconnected to the joint of the resistor R4 and NMOS transistor M4.

The sources of PMOS transistors M5 and M6, as a differential pair, areinterconnected, and the constant current source 12 is connected betweenthe joint of the differential pair and the input terminal IN.

The gates of NMOS transistors M7 and M8 are interconnected and the jointthereof is connected to the drain of NMOS transistor M8, whereby acurrent mirror circuit is formed. The sources of NMOS transistors M7 andM8 are each connected to the ground, the drain of NMOS transistor M7 isconnected to the drain of PMOS transistor M5 and the joint thereofserves as the output terminal of the differential amplifier circuit A2.In addition, the drain of NMOS transistor M8 is connected to the drainof PMOS transistor M6.

The voltage Vof applied to the source of PMOS transistor M5 isindicative of an offset voltage at the differential pair constituted ofPMOS transistors M5 and M6.

It may be added that the method of forming the offset voltage Vof is notnecessarily limited to the abovementioned method, but may alternativelybe selected from several known methods such as, for example, one withdifferent device size of the PMOS transistors M5 and M6.

With the abovementioned device configuration, the error amplificationcircuit A1 controls so that the partial voltage VFB coincides to thereference voltage Vref.

In the case when the output current io is smaller than a predeterminedcurrent value for exerting over-current protection, the drain current ofthe current detection transistor M2 is relatively small and the voltagedrop by the resistor R3 becomes smaller than the sum of the referencevoltage Vref and the offset voltage Vof.

As a result, PMOS transistor M6 is turned on and PMOS transistor M5 isturned off, this brings the drain voltage of PMOS transistor M5 toapproximately the ground potential.

Since NMOS transistor M4 is turned off and the drain voltage of PMOStransistor M4, i.e., the gate voltage of PMOS transistor M3, is broughtapproximately to the input voltage Vin, PMOS transistor M3 is turned offand no over-current protection is performed.

By contrast, when the output current io increases to be equal to, orlarger than, the predetermined current value for exerting over-currentprotection, the voltage drop by the resistor R3 is brought to be equalto the sum of the reference voltage Vref and the offset voltage Vof. Asa result, the drain voltage of PMOS transistor M5 increases and NMOStransistor M4 is turned on.

Since the source of NMOS transistor M4 is grounded, its drain voltagecan be decreased to approximately the ground potential. Therefore, PMOStransistor M3 can be sufficiently turned on even at low values of theinput voltage Vin.

By turning the PMOS transistor M3 on, the gate voltage of the outputtransistor M1 decreases, the increase in output current io issuppressed, and the output voltage Vo decreases.

Since the gate voltage of the PMOS transistor M5 decreases along withthe decrease in the output voltage Vo, the over-current protection canbe made operative for a small voltage drop by the resistor R3, and theoutput current io decreases with the decrease in the output voltage Vo.

When the output terminal OUT is short-circuited to the ground potential,the gate voltage of the PMOS transistor M6 becomes equal to offset thevoltage Vof.

During the short-circuit of the output terminal OUT, the magnitude ofthe output current io is equal to the product of the current through theresistor R3 multiplied by the ratio of current between the outputtransistor M1 and the current detection transistor M2. That is, themagnitude of short-circuit current can be set by the values of theresistance of resistor R3 and the offset voltage Vof.

Moreover, as described earlier, the amplitude of the drain voltage ofthe PMOS transistor M5, as the output voltage of the differentialamplifier circuit A2, is subjected to amplitude (or, magnitude)expansion so as to range from the ground potential to input voltage Vinby means of an inverter consisting of the NMOS transistor M4 andresistor R4.

As a result, the ON/OFF control of PMOS transistor M3 becomes feasibleat low values of the input voltage Vin and the over-current protectioncan be performed by controlling the output transistor M1.

Although the resistor R4 is used as the load of the NMOS transistor M4in FIG. 1, a constant current source may alternatively be used instead.

As describe herein above, the constant-voltage power supply circuitaccording to the present embodiment is configured such that a voltageoutput with small amplitude from the differential amplifier circuit A2is subjected to the amplitude expansion to be amplified to range fullyfrom the ground potential approximately to the input voltage Vin bymeans of the inverter consisting of the NMOS transistor M4 and resistorR4, and that the resulting voltage is subsequently input to the gate ofPMOS transistor M3 which is capable of directly controlling the outputtransistor M1.

Accordingly, the ON/OFF control of PMOS transistor M3 becomes feasibleat low input voltage Vin and the over-current protection can beperformed through the control of the output transistor M1.

FIG. 2 is a diagrammatic circuit diagram illustrating a constant-voltagepower supply circuit according to a second embodiment disclosed herein.

The components in the constant-voltage power supply circuit of FIG. 2that are similar to those of the power supply circuit described earlierin reference to FIG. 1 are shown with identical numericalrepresentations, and the description thereof is herein abbreviated forpurposes of clarity.

The constant-voltage power supply circuit of FIG. 2 has a devicestructure similar to the power supply circuit of FIG. 1, with theexception that the resistor R4 of FIG. 1 is eliminated, and that NMOStransistors M9 and PMOS transistors M10 and M11 are appended, wherebyNMOS transistors M7 and M4 constitutes a current mirror circuit and NMOStransistors M8 and M9 constitutes a further current mirror circuit.

Accordingly, there shown in FIG. 2 are a constant-voltage power supplycircuit 1 a and an over-current protection circuit 3 a in place of theconstant-voltage power supply circuit 1 and an over-current protectioncircuit 3 of FIG. 1, respectively.

Referring to FIG. 2, the constant-voltage power supply circuit 1 a isconfigured to convert an input voltage Vin input to an input terminalIN, into a predetermined constant voltage, and subsequently output as anoutput voltage Vo from an output terminal OUT. The constant-voltagepower supply circuit 1 a may be integrated to be a single IC device.

The constant-voltage power supply circuit 1 a includes at least aconstant-voltage circuit 2, and an over-current protection circuit 3 aconfigured to have the fold back current limiting capability, in thatthe current limiting capability is characterized by the function of theprotection circuit 3 a such that, in the case when an output current io,which is output from the output terminal OUT, is equal to or larger thana predetermined value of over-current protection current, the outputcurrent io is controlled to be suitably suppressed by decreasing theoutput voltage Vo.

The over-current protection circuit 3 a includes a differentialamplifier circuit A2 a, PMOS transistors M2, M3, M10, and M11, NMOStransistors M4 and M9, and a resistor R3.

In addition, the differential amplifier circuit A2 a includes PMOStransistors M5 and M6, NMOS transistors M7 and M8, and a constantcurrent source 12.

Still in addition, NMOS transistor M4 constitutes a first transistor,NMOS transistor M9 a second transistor, NMOS transistors M7 and M4 afirst current mirror circuit, NMOS transistors M8 and M9 a secondcurrent mirror circuit, and PMOS transistors M10 and M11 a third currentmirror circuit, respectively.

In respect to the over-current protection circuit 3 a, the sources ofPMOS transistors M5 and M6, as a differential pair, are interconnected,and the constant current source 12 is connected between the joint of thedifferential pair and the input terminal IN.

NMOS transistors M7 is connected between the drain of PMOS transistor M5and the ground, and the gate and drain of NMOS transistors M7 areinterconnected, whereby a diode is formed. Similarly, NMOS transistorsM8 is connected between the drain of PMOS transistor M6 and the ground,and the gate and drain of NMOS transistors M8 are interconnected,whereby a further diode is formed.

The joint between the drain of NMOS transistor M7 and the drain of PMOStransistor M5 serves as one of output terminals of the differentialamplifier circuit A2 a, and is connected to the gate of NMOS transistorM4.

In addition, the joint between the drain of PMOS transistor M6 and thedrain of NMOS transistor M8 serves as the other output terminal of thedifferential amplifier circuit A2 a, and is connected to the gate ofNMOS transistor M9.

There connected serially between the input terminal IN are PMOStransistors M10 and NMOS transistors M4, and also PMOS transistor M11and NMOS transistor M9. The third current mirror circuit is formedconsisting of PMOS transistors M10 and M11, in which the gates thereofare interconnected to form a joint, and which the joint is connected tothe drain of PMOS transistor M11.

In addition, the joint between PMOS transistors M10 and NMOS transistorM4 is connected to the gate of PMOS transistors M3.

When the output current io is smaller than a predetermined current valuefor exerting over-current protection with the abovementioned deviceconfiguration, the drain current of the current detection transistor M2is relatively small and the voltage drop by the resistor R3 becomessmaller than the sum of the reference voltage Vref and the offsetvoltage Vof.

As a result, PMOS transistor M6 is turned on while PMOS transistors M5is off, whereby the bias current ib from the constant current source 12is supplied entirely as the drain current for PMOS transistors M5. Thisdrain current for PMOS transistors M5 is transformed to that for NMOStransistor M9 by way of NMOS transistor M8.

Namely, since the drain current of NMOS transistor M9 is large whilethat of NMOS transistor M4 is minimal, the drain voltage of NMOStransistor M4 is brought approximately equal to the input voltage Vin.As a result, PMOS transistor M3 is turned off and no over-currentprotection is performed.

By contrast, when the output current io increases to be equal to, orlarger than, the predetermined current value for exerting over-currentprotection, the voltage drop by the resistor R3 is brought to be equalto the sum of the reference voltage Vref and the offset voltage Vof.Then, the drain current of PMOS transistor M5 increases and its drainvoltage decreases.

As a result, the drain voltage of NMOS transistor M4 increases and itsdrain voltage decreases.

Since the source of NMOS transistor M4 is grounded, its drain voltagecan be decreased to approximately the ground potential, and PMOStransistor M3 can be sufficiently turned on even at low values of theinput voltage Vin, as a result.

By turning the PMOS transistor M3 on, the gate voltage of the outputtransistor M1 is elevated, the increase in output current io issuppressed, and the output voltage Vo decreases.

Since the gate voltage of the PMOS transistor M5 also decreases alongwith the decrease in the output voltage Vo, the over-current protectioncan be made operative for a small voltage drop by the resistor R3, andthe output current io decreases with the decrease in the output voltageVo.

Incidentally, the output current io at the output voltage Vo zero, i.e.,short-circuit current, can be set in a manner similar to the methoddescribed earlier in reference to FIG. 1.

As describe herein above, the constant-voltage power supply circuitaccording to the second embodiment is configured such that a voltageoutput with small amplitude from the differential amplifier circuit A2 ais subjected to the amplitude expansion to be amplified so as to rangefully from the ground potential approximately to the input voltage Vinby means of the inverter consisting of NMOS transistors M4 and M9 andPMOS transistors M10 and M11, and that the resulting voltage issubsequently input to the gate of PMOS transistor M3 which is capable ofdirectly controlling the output transistor M1.

Accordingly, the ON/OFF control of PMOS transistor M3 becomes feasibleat low input voltage Vin and the over-current protection circuit can beoperated properly through the control of the output transistor M1.

Moreover, since the over-current protection circuit can be obtained inthe second embodiment having a gain smaller the first embodiment, thepresent circuit has a higher stability and can be operated stably in theconfiguration of simple phase compensation circuit.

In addition, the partial voltage VFB has been adapted to be input to thegate of PMOS transistor M5 in both the first and second embodiments.

However, since it is sufficient for a voltage proportional to the outputvoltage Vo to be input to that gate, the circuit may alternatively beconstructed such that another circuit is additionally formed forgenerating a voltage proportional to the output voltage Vo, and that aresulting proportional voltage is input to the gate of PMOS transistorM5.

It is apparent from the above description including the examplesdisclosed that the constant-voltage power circuit of the invention canoffer several advantages over similar power circuits previously known.

For example, the present constant-voltage power supply circuit isconfigured such that a voltage output with small amplitude from thedifferential amplifier circuit is subjected to the amplitude expansionto be amplified to range fully from the ground potential approximatelyto the input voltage by the inverter and PMOS transistors, and that theresulting voltage is subsequently input to the gate of one of the PMOStransistors which is capable of directly controlling the outputtransistor.

As a result, the over-current protection can be attained securely in apredetermined way even through low input voltages.

In addition, since the magnitude of the output current undershort-circuit conditions is equal to the product of the current throughthe resistor for current-voltage conversion circuit multiplied by theratio of current between the output transistor and the current detectiontransistor, the magnitude of short-circuit current can be set by thevalues of the resistance of the resistor and the offset voltage.

Together with the input circuit provided with offset voltages,therefore, the magnitude of short-circuit current can be adjusted withrelative ease in the present constant-voltage power supply circuit.

Still in addition, since the over-current protection circuit can beobtained in the abovementioned second embodiment having a gain smallerthe first embodiment, the former circuit has a higher stability and canbe operated stably in the configuration of simple phase compensationcircuit.

The fabrication of the constant-voltage power supply circuit set forthin the present description may be implemented using conventional generalpurpose microprocessors, programmed according to the teachings in thepresent specification, as will be appreciated to those skilled in therelevant arts. Appropriate software coding can readily be prepared byskilled programmers based on the teachings of the present disclosure, aswill also be apparent to those skilled in the relevant arts.

The present specification thus include also a computer-based productwhich may be hosted on a storage medium, and include instructions whichcan be used to program a microprocessor to perform a process inaccordance with the present disclosure. This storage medium can include,but not limited to, any type of disc including floppy discs, opticaldiscs, CD-ROMs, magneto-optical discs, ROMs, RAMs, EPROMs, EEPROMs,flash memory, magnetic or optical cards, or any type of media suitablefor storing electronic instructions.

While the invention has been described in conjunction with the preferredembodiments, including specific device components, and configurations,it is evident that many alternatives and variations will be apparent tothose skilled in the art. Accordingly, the preferred embodiments of theinvention as set forth herein are intended to be illustrative, notlimiting. Various changes may be made without departing from the spiritand scope of the invention as defined in the following claims.

1. A constant-voltage power supply circuit configured to perform atleast a first operation to input an input voltage to an input terminal,a second operation to convert said input voltage into a predeterminedconstant voltage, and a third operation to output said predeterminedconstant voltage as a first voltage output from an output terminal,comprising: an output transistor configured to output a first currentfrom said input terminal to said output terminal according to a signalinput to a control electrode; an output voltage control unit configuredto perform a fourth operation to generate said predetermined referencevoltage, a fifth operation to generate a second voltage proportional tosaid first voltage, a sixth operation to amplify a difference betweensaid predetermined reference voltage and said second voltageproportional to said first voltage to obtain an amplified difference,and a seventh operation to input said amplified difference to saidcontrol electrode; an over-current protection circuit configured toperform a first control operation on said output transistor, in a casewhen said first current from said output terminal reaches apredetermined current value or greater for a rated output voltage, todecrease said first current as well as said first voltage; and a secondcontrol operation on said output transistor, in a case when said firstvoltage from said output terminal is decreased to a ground potential, tooutput a predetermined short-circuit current from said output terminal;said over-current protection circuit comprising (a) a current detectiontransistor configured to perform an eighth operation to detect saidfirst current from said output transistor, and a ninth operation tooutput a second current proportional to said first current from saidoutput transistor, a control electrode of said current detectiontransistor being connected to the control electrode of said outputtransistor, a current input terminal of said current detectiontransistor being connected to said input terminal as well as said outputtransistor; (b) a current-voltage conversion circuit configured toconvert said second current from said current detection transistor intoa third voltage to obtain a converted voltage; (c) an erroramplification circuit, first and second input terminals of said erroramplification circuit being input with said second current proportionalto said first current output from said output transistor and saidconverted voltage, respectively; (d) an amplitude expander configured toexpand an amplitude of a fourth voltage output from said erroramplification circuit, so as to range from the ground potentialapproximately to said input voltage; and (e) a control circuitconfigured to control an operation of said output transistor accordingto an output signal from said amplitude expander.
 2. Theconstant-voltage power supply circuit according to claim 1, whereininput and output terminals of said amplitude expander are connected to acorresponding terminal of said error amplification circuit and saidcontrol circuit, respectively, to thereby form an inverter circuit. 3.The constant-voltage power supply circuit according to claim 1, saiderror amplification circuit comprising first and second transistorsconfigured to form first and second halves of a differential pair tothereby form first and second diodes as first and second loads of saidfirst and second transistors, respectively; said amplitude expandercomprising (i) a third transistor to form a first current mirror circuitin combination with said first transistor, an control electrode of saidfirst transistor being input with said second current proportional tosaid first current; (ii) a fourth transistor to form a second currentmirror circuit in combination with said second transistor, an controlelectrode of said second transistor being input with said convertedvoltage; and (iii) a third current mirror circuit configured to supply athird current from said fourth transistor to said third transistor,wherein said control circuit is configured to control an operation ofsaid output transistor according to a fifth voltage at a junctionbetween said third transistor and said third current mirror circuit. 4.The constant-voltage power supply circuit according to claim 1, whereinsaid error amplification circuit includes an input circuit, said inputcircuit being provided with an offset voltage such that a value of saidpredetermined short-circuit current is obtained as positive.
 5. Theconstant-voltage power supply circuit according to claim 1, wherein saiderror amplification circuit includes an input circuit, said inputcircuit being provided with an offset voltage such that a value of saidpredetermined short-circuit current is obtained as positive.
 6. Theconstant-voltage power supply circuit according to claim 1, wherein saiderror amplification circuit includes an input circuit, said inputcircuit being provided with an offset voltage such that a value of saidpredetermined short-circuit current is obtained as positive.
 7. Aconstant-voltage power supply circuit configured to perform at least afirst operation to input an input voltage to an input terminal, a secondoperation to convert said input voltage into a predetermined constantvoltage, and a third operation to output said predetermined constantvoltage as a first voltage output from an output terminal, comprising:means for outputting a first current from said input terminal to saidoutput terminal according to a signal input to a control electrode;means for generating said predetermined reference voltage, generating asecond voltage proportional to said first voltage, amplifying adifference between said predetermined reference voltage and said secondvoltage proportional to said first voltage to obtain an amplifieddifference, and inputting said amplified difference to said controlelectrode; and over-current protection means, including firstcontrolling means for controlling said means for outputting, in a casewhen said first current from said output terminal reaches apredetermined current value or greater for a rated output voltage, todecrease said first current as well as said first voltage; secondcontrolling means for controlling said means for outputting, in a casewhen said first voltage from said output terminal is decreased to aground potential, to output a predetermined short-circuit current fromsaid output terminal; means for detecting said first current from saidoutput transistor; means for outputting a second current proportional tosaid first current from said output transistor; means for convertingsaid second current from said current detection transistor into a thirdvoltage to obtain a converted voltage; error amplification means;amplitude expander means for expanding an amplitude of a fourth voltageoutput from said error amplification means to range from the groundpotential approximately to said input voltage; and third control meansfor controlling an operation of said means for outputting according toan output signal from said amplitude expander mean.
 8. Theconstant-voltage power supply circuit according to claim 7, wherein saiderror amplification means includes input means, said input means beingprovided with an offset voltage such that a value of said predeterminedshort-circuit current is obtained as positive.